And look at the price ! with no gearbox, clutch or flywheel

You'll buy a 300bhp Lexus LS400 complete car or sub £1k. Sell some bits and weigh in for scrap. Engine will almost be free then.
£1k to sort some sort of gearbox, possibly including clutch at that.

It's a relatively compact engine, absolutely bulletproof and very very tuneable.

The M5 will be a great engine....but when something goes wrong, it could be another £3k to fix it.